The present invention relates in general to a method and a device for speed control of a bicycle. More particularly, the present invention relates to a method and a device for speed control of a bicycle which employ a synchronous control mechanism for bicycle front and rear derailleurs, which mechanism aimed at synchronously controlling both front and rear derailleurs to achieve predetermined sequential combinations of front and rear gears of front and rear gear systems of the bicycle, respectively, to enable a gradual and smooth increase/decrease of speed of the bicycle.
A Bicycle includes a front gear rotated via pedals by a bicyclist, the rotational movement of the front gear is transmitted via a chain to a rear gear, itself connected to the rear wheel of the bicycle. The number of rotations performed by the rear gear (and thus the rear wheel) per a single rotation of the front gear (i.e., pedals) equals the ratio, m/n, between the number of teeth (or radius) of the front gear, m, and the number of teeth (or radius) of the rear gear, n.
With reference now to FIG. 1. In order to enable more versatile and efficient riding, a speed controlled bicycle has been developed in which numerous discrete m/n ratios can be selected by the bicyclist. Thus, a speed controlled bicycle includes a front 20a and a rear 20b gear systems, at least one of the systems has at least two parallel gears each of a different radius (i.e., a different number of teeth). Typically the front gear system 20a includes two or three front gears 28a-c of different sizes and the rear gear system 20b includes five or six rear gears 22a-f of different sizes (i.e., 10, 12, 15 or 18 different gear combinations). A speed controlled bicycle further includes a front and a rear speed control devices, 24a and 24b, respectively, each includes a front derailleur 26a or a rear derailleur 26b, and derailleur operating means, 19a and 19b, respectively, to select a combination of a selected front gear and a selected rear gear to be engaged by the chain 18, to obtain a suitable and/or desired m/n ratio. Rear derailleur 26b typically includes means such as a tension pulley to maintain the chain at a predetermined range of tension (not shown).
For gear selection, each of the derailleur operating means 19a and 19b typically includes guiding means (known in the art as shifters) 21a and 21b, respectively. Each of guiding means 21a and 21b has segments 23a-c and 25a-f, respectively. Each of segments 23a-c and 25a-f has a predetermined gradually increasing or decreasing line-distance from the front 26a and rear 26b derailleurs, respectively. Thus, by separately guiding members 27a and 27b, to which members 27a and 27b connected are lines (wires, or as recently introduced to the art, hydraulic lines) 29a and 29b, themselves connected to front 26a and rear 26b derailleurs, respectively, along guiding means 21a and 21b, the locations of each of the front 26a and rear 26b derailleurs is separately and mechanically independently determined, to enable a separate selection of a specific front (28a in the example of FIG. 1) and a specific rear (22a in the example of FIG. 1) gears to be engaged by chain 18. It should be noted that prior art derailleurs operating means 24a and 24b are each mechanically independently operated by the bicyclist.
As for the simple bicycle described above, the front gear system of a speed controlled bicycle hereinabove described is forwardly rotated via pedals operated by a bicyclist, which forward rotation is transmitted onto the rear gear system by the chain, resulting in a forward rotation of the rear gear system and of the rear wheel therewith. Similarly to as described, the number of rotations performed by the rear wheel per a single rotation performed by the front gear system equals the ratio m/n, wherein m is the number of teeth (or radius) of a selected front gear engaged by the chain in the front gear system and n is the number of teeth (or radius) of a selected rear gear engaged by the chain in the rear gear system. Speed control is thus achieved by changing the m/n ratio, via mechanically independently selecting combinations of a gear of the front gear system via the front speed control device and a gear of the rear gear system via the rear speed control device.
The number of teeth in front and rear gears of a typical bicycle having three front gears (marked as `f`) in the front gear system and six rear gears (marked as `r`) in the rear gear system, and the m/n ratio for each of the eighteen combinations of front and rear gears, are given in Table 1 below.
TABLE 1 ______________________________________ # of teeth m r28 r24 r21 r18 r16 r14 ______________________________________ f28 1.00 1.17 1.33 1.56 1.75 2.00 f38 1.36 1.58 1.81 2.11 2.37 2.71 f48 1.71 2.00 2.29 2.77 3.00 3.42 ______________________________________
As shown in Table 1, the lowest speed is achieved selecting the smallest front gear, f28, and the largest rear gear, r28, wherein the m/n ratio equals 1.00. In this case, the number of rotations performed by the rear wheel per a single rotation performed by the front gear is 1.00. As further shown in Table 1, the highest speed is achieved selecting the largest front gear, f48, and the smallest rear gear, r14, wherein the m/n ratio equals 3.42. In this case, the number of rotations performed by the rear wheel per a single rotation performed by the front gear is 3.42.
Apparently a bicycle with the parameters described in Table 1, has eighteen (3.times.6) different speeds. For a gradual increase/decrease of speed, the bicyclist should select front and rear gear combinations with gradually increased/decreased m/n ratios, respectively.
Table 2 presents grades (1 through 18) for increasing m/n ratios as are calculated in Table 1, wherein 1 represents the lowest ratio (i.e., 1.00 in the given example) and 18 represents the highest ratio (i.e., 3.42 in the given example).
TABLE 2 ______________________________________ # of teeth m r28 r24 r21 r18 r16 r14 ______________________________________ f28 1 2 3 5 8 11 f38 4 6 9 12 14 15 f48 7 10 13 16 17 18 ______________________________________
Table 3 presents the combined number of teeth (n+m) for each of the eighteen combinations, which combined number being an indication of the tension imposed on the tension pulley, wherein the higher the combined number is, the higher the tension is and vice versa.
TABLE 3 ______________________________________ # of teeth m r28 r24 r21 r18 r16 r14 ______________________________________ f28 56 52 49 46 44 42 f38 66 62 59 56 54 52 f48 76 72 69 66 64 62 ______________________________________
From this description it is apparent that following the eighteen sequentially increasing/decreasing m/n ratios have few drawbacks as follows.
First, as best seen in Tables 1 and 2, gradually increasing/decreasing the m/n ratios involves a non linear, neither smooth increase/decrease of the ratio, wherein few (i.e., two or more) sequential combinations (e.g., from f28/r18 to f38/r24; from f28/r16 to f38/r21) have similar m/n values, some even have identical values (e.g., from f48/r24 to f28/r14), whereas other sequential combinations are characterized by big differences in their m/n ratios (e.g., from f38/r28 to f28/r18; from f38/r16 to f38/r14), resulting in a non smooth operation.
Second, as best seen in table 2, following the 1 through 18 combinations involves, in many instances, shifting a gear in both front and rear gear systems and furthermore, involves gear shifting between non-adjacent gears in the same gear system (i.e., front and/or rear). For example, shifting from combination 10 to combination 11 involves shifting from the smallest rear gear, r14, to the largest, r28, in the rear gear system and from the smallest gear, f28, to the largest gear, f48, in the front gear system. Therefore, this type of operation is impractical for both the average and professional bicyclist.
Third, as best seen in Table 3, following combinations 1 through 18 involves high changes of tension imposed on a spring being part of the tension pulley of the rear derailleur. For example while moving from combination f28/r18 to combination f38/r24, the combined number of teeth changes from 46 to 62.
And finally, since the largest front gear and the largest rear gear are positioned opposite in relation to the frame of the bicycle, typically the largest front gear is away from the frame and the largest rear gear is close to the frame, few combinations characterized by a highly diagonalized position of the chain are not recommended due to the high friction thus formed between the gears and the chain. In the given example these combinations are f48/r28, f38/r28, f28/r14 and f38/r14, underlined in Table 2.
Reviewing the data presented in Table 1 above reveals that by selecting only 10 combinations, A through J, presented in Table 4 below, overcomes all of the above listed limitations.
TABLE 4 ______________________________________ # of teeth m r28 r24 r21 r18 r16 r14 ______________________________________ f28 A B C f38 D E f48 F G H I J ______________________________________
Thus, following the A through J combinations have advantages: (1) it provides a gradual and smooth increase in the m/n ratio; (2) only twice the bicyclist is required to shift gears both at the front and rear gear systems, that is while moving from combination C to combination D and from combination E to combination F; (3) in all gear shifts only adjacent gears are involved; (4) the combined number of teeth ranges from 49 (combination C) to 62 (combination J) thus limiting the tension range imposed on the spring of the tension pulley of the rear derailleur, thereby prolonging its life span, and (5) it avoids chain diagonalization.
However, following the A through J combinations requires (1) calculating this root as was done hereinabove in Tables 1 through 4 (2) memorizing thus calculated root; (3) employing both a front and a rear speed control devices to be in some cases operated mechanically independently, yet as simultaneously as possible, by the bicyclist.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and a device providing a synchronous control mechanism for bicycle front and rear derailleurs, which mechanism is for synchronously controlling both the front and the rear derailleurs, to achieve predetermined sequential combinations of front and rear gears for obtaining a gradual and smooth increase/decrease of speed of a speed controlled bicycle.